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Having a problem with an aquarium. I had cycled the tank to where nitrite, nitrate, and ammonia levels were 0. PH is/was 7.6-7.8. GH and KH is between 50-100 ppm.

First, I can’t find a scale to read GH and KH values in ppm. I always get single digit numbers like 4.5-7 as “ideal values.” Is that ppm? Or are they measuring in another scale? How do I convert 50-100 ppm to whatever scale that is?

Second, my son wanted neons and I thought 7.8 was too high for them, but the petsmart people convinced us 7.8 was ok. 4 days later, they all died. What caused their death?

High KH/GH? High pH? I introduced them to the water over about an hour after warming them up to the temp in the tank, which is a constant 78 F.

This morning, I did a 5 gallon water change in our 25 gallon tank and we want to go back to the store to bitch about the fish, but I don’t want to kill any more fish, regardless of them being neons or something else.

Tested water before water change and ammonia was below .25 ppm and nitrites were up to <5ppm. This doesn't seem like unlivable conditions, especially since all the other fish are still alive and doing well. What is odd is they always die at night. Not during the day. What is killing my fish?